Presentation on theme: "Steep unstable or moderately stable slopes are a common concern in watersheds on Vancouver Island. Urban development, logging, road building and exploration."— Presentation transcript:
Steep unstable or moderately stable slopes are a common concern in watersheds on Vancouver Island. Urban development, logging, road building and exploration may contribute to the occurrence of landslides. The negative impacts of landslides may include: Threats to life, property or infrastructure. Total (or partial) devastation of stream reaches or tributaries in a watershed. Complete (or partial) removal of vegetation (including habitat features) from a site. Sediment source (chronic, catastrophic, or one time) to streams, thereby having a detrimental effect on habitat. Destabilization of stream reaches, destruction of habitat, emplacement of fish barriers. Visual concerns. Does Vancouver Island have landslides? Landslides: Sediment, debris and rock moving downslope under the influence of gravity. Landslides range from very small (<10s of m 3 ) to very large (millions of m 3 ). YES!
FallSlumpFlowSlide Landslides are grouped into four major types: Further classification is based on material type (rock, earth, debris). Example: debris slide, rock fall. What causes landslides? Common trigger mechanisms are: precipitation earthquakes human activities temperature change wave erosion river erosion time
Parksville Nanaimo Duncan Pt. Alberni Estevan Lowlands Fiordlands South Vancouver Island Ranges Nanaimo Lowlands Fiordlands: Wetter (3000 mm of rain) Steep slopes; thinner soils and bedrock, lower slopes have glacial and ice contact sediments Windward facing Many landslides Common types: debris slides and flows Common causes: natural, road building, logging. South Vancouver Island Ranges: Much of the precipitation falls as snow Steep and rugged; thinner soils and bedrock, lower slopes have glacial and ice contact sediments Mid and lower slopes may experience rain on snow (compounds precipitation) Many landslides Common types: rock falls, debris slides and debris flows Common causes: natural, road building, logging. Nanaimo Lowlands: Relatively dry (1500 mm of rain) Subdued topography Deep glacial, alluvial and ice contact deposits Focussed urban development Fewer landslides, concentrated along rivers and shorelines Higher unit-area costs in terms of mitigation Common types: slumps Common causes: undercutting, water diversion, excavation Do we have landslides near Nanaimo? Yes! Natural and man-induced, in urban and remote settings. Landslides are a common feature on Vancouver Island. Precipitation is a major cause of landslides and generally speaking, wetter slopes are more susceptible than drier. Snowmelt is not a big concern in this part of the world, however, occasionally we get a rain on snow event, where rain melts snow, compounding the hydrological effect. These storms are sometimes referred to locally as a pineapple express. They may result in many landslides. Drier areas generally have fewer natural landslides. WE
Natural landslides are common given the right conditions: Steep slopes A suitable trigger mechanism (rain, earthquake, time) Loose surficial sediments or unstable bedrock The smallest landslides occur frequently; several per year, however, they tend not to be noticed. Talus slopes are formed this way via many small rockfalls. Large landslides, may only occur once in hundreds of years (for a given watershed), and may fundamentally change the shape of the basin. These are often rockfall/avalanches. Debris slides and debris flows may occur in a watershed may occur every few years, depending on the conditions.
Forestry and landslides Forestry is a major industry on Vancouver Island. Logging and building roads on steep slopes often results in landslides. On Vancouver Island, the impacts can be considerable particularly to fish bearing streams. The likelihood of landslide occurrence depends on the underlying terrain, climatic conditions and care shown by the local companies. Roads undercut or overload the slope. Logging removes supportive root networks and scars the slope. The role of vegetation For shallow landslides such as debris slides and debris flows, the intact forest acts as both an additional stress on the slope (weight) and as a strengthening agent. Roots knit together loose soil and debris, and evapotranspiration removes moisture from the soil, particularly in the spring and fall months. Debris slide in a cutblock Road related debris slide and debris flow (below road)
Landslides where we live: In the inevitable attempts for a better view, many of us place ourselves at risk by building a house or structure too close to a riverbank or an eroding ocean side cliff. Often the re-routing of water from houses, roads and through sewers and storm pipes make matters worse. Pipe left from water exiting sediment Trees and grass are deposit of recent slump River may also erode bank over longer period house House threatened by several riverside slumps. The main cause of failure is from the collapse of high sand banks due to large amounts of water exiting through sediment. A longer term concern would be erosion of the harder till layer below, which in time would also result in collapse of the bank.
What are we doing about landslides? The Forest Practices Code includes legislation aimed at reducing the impacts and numbers of landslides from forestry activities. Local development setback bylaws attempt to provide a zone of safety for private landowners.